Nuclear fuel elements are known which are adapted to be grouped in the form of parallel clusters or assemblies constituting, by their juxtaposition, the core of a nuclear reactor. Such fuel elements comprise a tubular sheath closed at both ends, composed of a material resisting creep and corrosion under the action of the coolant at high pressure and high temperature of the nuclear reactor, and enclosing pellets stacked in the axial direction of the sheath, said pellets being generally in sintered form and of a nuclear fuel material such as uranium oxide or a mixed uranium and plutonium oxide.
The sheath of the nuclear fuel element has in particular the function of preventing the escape of the gaseous product of fission in the coolant fluid, and the contact between the fuel material and the coolant which would result in chemical reactions. The sheath also performs the mechanical function of maintaining the stack of pellets and resisting the pressure difference between the interior and exterior of the fuel element.
The long-term performance of nuclear fuel elements, and in particular those employed in a nuclear reactor which is cooled and moderated with light water depends on a phenomenon known by the name of pellet-sheath interaction. This interaction results from the combination of a mechanical action, namely, the stressing of the sheath upon swelling and expansion of the pellets under irradiation, and a chemical action due in particular to the products of fission liberated by the pellets under irradiation.
Various solutions have been proposed for limiting to the extent possible the pellet-sheath interaction or the consequences thereof on the behavior of the fuel elements.
For example it has been proposed in French Patent Application No. 2 551 905, to place inside the sheath of the fuel element an open tube for maintaining the stack of pellets in at least a part of its length. This tube permits separating the sheath from the stack and is separated from the sheath by a radial gap of sufficient extent to retard the contact between the tube and the sheath when the fuel pellets swell under irradiation.
The production of such a fuel element presents difficulties in that this element is of great length, for example on the order of 40 meters, as in the case of nuclear reactors cooled with pressurized water. It is indeed difficult to ensure manufacture and mounting within the sheath of a tube of great length in such manner as to provide a small and constant radial clearance throughout the length of the tube between the latter and the inner surface of the sheath.
The production of a stack of pellets within the tube of great length is also a lengthy operation which is difficult to carry out and requires pellets whose diameter is slightly less than the inside diameter of the tube, the presence of a certain radial clearance by construction being indispensable. This radial clearance reduces the efficiency of the thermal exchanges between the pellets and the sheath so that it is necessary to increase the temperature of the pellets in operation in order to maintain the thermal flux at the required value.
Furthermore, it may be necessary to employ nuclear fuel elements which comprise regions whose composition differs depending on the position of these regions in the axial direction of the fuel element.
For example, it may be desirable to employ fuel elements whose enrichment of pellets varies in the axial direction of the rod or fuel elements whose regions close to the ends comprise pellets composed of a material such as a fertile material for example a depleted or natural uranium oxide UO.sub.2 or thorium oxide ThO.sub.2 for constituting the axial covers at the upper and lower partsof the core.
It may also be necessary to employ fuel elements comprising, in the central part thereof, elements termed burnable poisons capable of absorbing the neutrons. Such burnable poison elements may, for example, be in the form of small plates each of which is interposed between two successive nuclear fuel pellets
It is clear that the manufacture of such composite fuel elements is much more complex than the manufacture of homogeneous fuel elements, since pellets or plates of different types must be introduced in the sheath of the fuel element in a predetermined number and order. It is also necessary to have available stocks of pellets of different types, which complicates the management of the fuel producing factory.